1. Field of the Invention
The present invention relates to magnetic particles suitable for a magnetic recording medium, a method of producing the same and a magnetic recording medium comprising a magnetic layer that contains the magnetic particles.
2. Description of the Related Art
Reducing particle sizes is essential for enhancing magnetic recording density. For example, when the weight of ferromagnetic body is the same, noise levels are decreased by reducing the particle size in magnetic recording media which are widely used for video tapes, computer tapes and disks.
The reduction in the size of the magnetic particles results in superparamagnetism of the particles due to thermal fluctuations, and the particles cannot be used for a magnetic recording medium. Accordingly, there have been studies on materials such as FePt having a high crystal magnetic anisotropy. However, these magnetic bodies contain platinum and the cost thereof becomes high.
As the magnetic materials having a high crystal magnetic anisotropy comparable to that of FePt magnetic particles, SmCo, NdFeB, SmFeN and the like have been known. However, in order to obtain these materials, in general, thermal treatment of the particles at temperatures of 500° C. or higher is required. When the thermal treatment of the particles is carried out at such high temperatures, the particles are fuse-coalesced to form polycrystals or coarse particles, so that the particles are not suitable for use in magnetic recording media.
The magnetic particles composed of NdFeB are produced in such a manner that a quenched thin band containing NdFeB crystals having a particle size of 5 to 50 nm is formed by a fused metal quenching method, and the quenched thin band is mechanically pulverized. These magnetic particles are used for a bond magnet or the like. The size of the magnetic particles pulverized mechanically is larger of the order of microns and the particles are not suitable for use in magnetic recording media. Further, in the particles, an Nd—Fe phase is segregated at the particle interface of the NdFeB crystals having a particle size of 5 to 50 nm, and there is a drawback that the crystals are susceptible to corrosion.
A method for synthesizing magnetic particles in a liquid phase followed by heating the particles in a hydrogen gas stream to reduce the particles is disclosed, for example, in Japanese Patent Application Laid-Open (JP-A) Nos. 2001-181754, 2002-50509 and 2002-121027. However, by using this method, it is difficult to obtain particles having a homogeneous composition. There is a problem such that the particles tend to be melt-coalesced by heating after nanometer-sized particles are synthesized.